Signal transduction-mediated CYP1A1 induction by omeprazole in human HepG2 cells

Effects of Chemicals in Reporter Gene Bioassays with Different Metabolic Activities Compared to Baseline Toxicity

High-throughput cell-based bioassays are used for chemical screening and risk assessment. Chemical transformation processes caused by abiotic degradation or metabolization can reduce the chemical concentration or, in some cases, lead to the formation of more toxic transformation products. Unaccounted loss processes may falsify the bioassay results. Capturing the formation and effects of transformation products is important for relating the in vitro effects to in vivo. Reporter gene cell lines are believed to have low metabolic activity, but inducibility of cytochrome P450 (CYP) enzymes has been reported. Baseline toxicity is the minimal toxicity a chemical can have and is caused by the incorporation of the chemical into cell membranes. In the present study, we improved an existing baseline toxicity model based on a newly defined critical membrane burden derived from freely dissolved effect concentrations, which are directly related to the membrane concentration. Experimental effect concentrations of 94 chemicals in three bioassays (AREc32, ARE-bla and GR-bla) were compared with baseline toxicity by calculating the toxic ratio (TR). CYP activities of all cell lines were determined by using fluorescence-based assays. Only ARE-bla showed a low basal CYP activity and inducibility and AREc32 showed a low inducibility. Overall cytotoxicity was similar in all three assays despite the different metabolic activities indicating that chemical metabolism is not relevant for the cytotoxicity of the tested chemicals in these assays. Up to 28 chemicals showed specific cytotoxicity with TR > 10 in the bioassays, but baseline toxicity could explain the effects of the majority of the remaining chemicals. Seven chemicals showed TR < 0.1 indicating inaccurate physicochemical properties or experimental artifacts like chemical precipitation, volatilization, degradation, or other loss processes during the in vitro bioassay. The new baseline model can be used not only to identify specific cytotoxicity mechanisms but also to identify potential problems in the experimental performance or evaluation of the bioassay and thus improve the quality of the bioassay data.

Dibenzyl trisulfide binds to and competitively inhibits the cytochrome P450 1A1 active site without impacting the expression of the aryl hydrocarbon receptor

The toxicological manifestation of many pollutants relies upon their binding to the aryl hydrocarbon receptor (AHR), and it follows a cascade of reactions culminating in an elevated expression of cytochrome P450 (CYP) 1 enzymes. CYP1A1 and CYP1B1 are associated with enhanced carcinogenesis when chronically exposed to certain polyaromatic hydrocarbons, and their inhibition may lead to chemoprevention. We evaluated dibenzyl trisulfide (DTS), expressed in the ethnomedical plant, Petiveria alliacea, for such potential chemoprevention. Using recombinant human CYP1A1 and CYP1B1 bactosomes on a fluorogenic assay, we first demonstrated that DTS moderately inhibited both enzymes with half maximal inhibitory concentration (IC50) values of 1.3 ± 0.3 and 1.7 ± 0.3 μM, respectively. Against CYP1A1, DTS was a reversible, competitive inhibitor with an apparent inhibitory constant (Ki) of 4.55 ± 0.37 μM. In silico molecular modeling showed that DTS binds with an affinity of -39.8 kJ·mol-1, situated inside the binding pocket, approximately 4.3 Å away from the heme group, exhibiting interactions with phenylalanine residue 123 (Phe-123), Phe-224, and Phe-258. Lastly, zebrafish (Danio rerio) embryos were exposed to 0.08-0.8 μM DTS from 24 to 96 h post fertilization (hpf) with the in vivo ethoxyresorufin-O-deethylase (EROD) assay, and, at 96 hpf, DTS significantly suppressed EROD CYP1A activity in a dose-dependent manner, with up to 60% suppression in the highest 0.8 μM exposure group. DTS had no impact on gene transcription levels for cyp1a and aryl hydrocarbon receptor 2 (ahr2). In co-exposure experiments, DTS suppressed CYP1A activity induced by both B[a]P and PCB-126, although these reductions were not significant. Taken together, these results demonstrate that DTS is a direct, reversible, competitive inhibitor of the carcinogen-activating CYP1A enzyme, binding in the active site pocket close to the heme site, and shows potential in chemoprevention.

COX-2 mRNA is increased in oesophageal mucosal cells by a proton pump inhibitor

BACKGROUND

Barrett's oesophagus develops in some individuals with gastro-oesophageal reflux and is the precursor to oesophageal adenocarcinoma. Proton pump inhibitors (PPIs) suppress gastric acid production and are used to treat reflux. Clinical trials suggest that cyclooxygenase (COX) inhibitors might prevent oesophageal cancer, although PPIs could offset this by increasing COX-2 expression in Barrett's oesophagus. To investigate this, we evaluated the impact of a PPI on COX expression in oesophageal mucosal cells.

METHODS

The effect of the PPI esomeprazole on COX-1 and COX-2 mRNA levels in oesophageal cells was determined. Oesophageal cell lines OE33 (adenocarcinoma-derived) and HET-1A (immortalized squamous cells) and a control intestinal cell line HT29 (colon carcinoma) were treated for 24 h, with increasing concentrations of the esomeprazole.

RESULTS

COX-2, but not COX-1, mRNA levels dose-dependently increased in OE33 and HET-1A cells versus esomeprazole concentration. COX-2 mRNA levels did not increase in HT29 cells.

CONCLUSIONS

Exposure to esomeprazole increases COX-2 mRNA in oesophageal cells. This might contribute to the lack of benefit for COX inhibitors for oesophageal cancer prevention in recent clinical studies.

Potential health-modulating effects of isoflavones and metabolites via activation of PPAR and AhR

Isoflavones have multiple actions on cell functions. The most prominent one is the activation of estrogen receptors. Other functions are often overlooked, but are equally important and explain the beneficial health effects of isoflavones. Isoflavones are potent dual PPARα/γ agonists and exert anti-inflammatory activity, which may contribute to the prevention of metabolic syndrome, atherosclerosis and various other inflammatory diseases. Some isoflavones are potent aryl hydrocarbon receptor (AhR) agonists and induce cell cycle arrest, chemoprevention and modulate xenobiotic metabolism. This review discusses effects mediated by the activation of AhR and PPARs and casts a light on the concerted action of isoflavones.

Role of signalling systems in the effects of dietary factors on the expression of mammalian CYPs

Changes in mammalian diets alter the hepatic expression of CYP drug-metabolising enzymes and endobiotic oxidases. Thus, dietary constituents may significantly influence the duration of action of chemicals in tissues. Recent improvements in the mechanistic information on the regulation of constitutive and inducible expression of CYPs has facilitated our understanding as to how dietary factors modulate expression. Altered regulation appears to occur either by direct activation of transcription factors or by indirect modulation of signal transduction pathways. For example, dietary lipid directly activates PPAR-alpha, or other nuclear hormone receptors, to elicit CYP induction, and vitamin A deficiency downregulates the growth hormone-responsive CYP2C11 by perturbing Janus kinase-signal transducers and activators of transcription signalling. This article focuses on the present understanding of the regulation of CYP genes by dietary nutrients.

Database of traditional Chinese medicine and its application to studies of mechanism and to prescription validation

BACKGROUND AND PURPOSE

Traditional Chinese Medicine (TCM) is widely practised and is viewed as an attractive alternative to conventional medicine. Quantitative information about TCM prescriptions, constituent herbs and herbal ingredients is necessary for studying and exploring TCM.

EXPERIMENTAL APPROACH

We manually collected information on TCM in books and other printed sources in Medline. The Traditional Chinese Medicine Information Database TCM-ID, at http://tcm.cz3.nus.edu.sg/group/tcm-id/tcmid.asp, was introduced for providing comprehensive information about all aspects of TCM including prescriptions, constituent herbs, herbal ingredients, molecular structure and functional properties of active ingredients, therapeutic and side effects, clinical indication and application and related matters.

RESULTS

TCM-ID currently contains information for 1,588 prescriptions, 1,313 herbs, 5,669 herbal ingredients, and the 3D structure of 3,725 herbal ingredients. The value of the data in TCM-ID was illustrated by using some of the data for an in-silico study of molecular mechanism of the therapeutic effects of herbal ingredients and for developing a computer program to validate TCM multi-herb preparations.

CONCLUSIONS AND IMPLICATIONS

The development of systems biology has led to a new design principle for therapeutic intervention strategy, the concept of 'magic shrapnel' (rather than the 'magic bullet'), involving many drugs against multiple targets, administered in a single treatment. TCM offers an extensive source of examples of this concept in which several active ingredients in one prescription are aimed at numerous targets and work together to provide therapeutic benefit. The database and its mining applications described here represent early efforts toward exploring TCM for new theories in drug discovery.

Effect of Omeprazole on Gastric Adenosine A1 and A2A Receptor Gene Expression and Function

Adenosine has been shown to inhibit immunoreactive gastrin (IRG) release and to stimulate somatostatin-like immunoreactivity (SLI) release by activating adenosine A(1) and A(2A) receptors, respectively. Since the synthesis and release of gastrin and somatostatin are regulated by the acid secretory state of the stomach, the effect of achlorhydria on A(1) and A(2A) receptor gene expression and function was examined. Omeprazole-induced achlorhydria was shown to suppress A(1) and A(2A) receptor gene expression in the antrum and corporeal mucosa, but not in the corporeal muscle. Omeprazole treatment produced reciprocal changes in A(1) receptor and gastrin gene expression, and parallel changes in A(2A) receptor and somatostatin gene expression. The localization of A(1) and A(2A) receptors on gastrinsecreting G-cells and somatostatin-secreting D-cells, respectively, suggests that changes in adenosine receptor expression may modulate the synthesis and release of gastrin and somatostatin. Thus, the effect of omeprazole on adenosine receptor-mediated changes in IRG and SLI release was also examined in the vascularly perfused rat stomach. After omeprazole treatment, the A(1) receptor-mediated inhibition of IRG and SLI release induced by N(6)-cyclopentyladenosine (A(1) receptor-selective agonist) was not altered, but the A(2A) receptor-mediated augmentation of SLI release induced by 2-p-(2-carboxyethyl-)phenethylamino-5'-N-ethylcarboxamidoadenosine (A(2A)-selective agonist) was significantly attenuated. These findings agree well with the corresponding omeprazole-induced decrease in antral A(2A) receptor mRNA expression. Overall, the present study suggests that adenosine receptor gene expression and function may be altered by omeprazole treatment. Acid-dependent changes in adenosine receptor expression may represent a novel purinergic regulatory feedback mechanism in controlling gastric acid secretion.

The role of protein tyrosine kinases in CYP1A1 induction by omeprazole and thiabendazole in rat hepatocytes

Benzimidazoles compounds like omeprazole (OME) and thiabendazole (TBZ) mediate CYP1A1 induction differently from classical aryl hydrocarbon receptor (AhR) ligands, 3-methylcholanthrene (3-MC) and 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). To clarify the involvement of an intracellular signal pathway in CYP1A1 induction by OME and TBZ, the TBZ, OME and 3-MC signal-transducing pathways were compared by using specific protein tyrosine kinase inhibitors in primary culture of rat hepatocytes. The effect of OME and TBZ (75-250 microM) on cytochrome P450 1A1 (CYP1A1) expression was therefore studied in primary cultures of rat hepatocytes after 24 h, 48 h and 72 h of exposure. Both compounds provoked a dose- and time-dependent increase in CYP1A1 (EROD activity, protein and mRNA levels), but OME was less effective at all the concentrations and times tested. The mechanism of benzimidazole-mediated induction of CYP1A1 was investigated by comparison with 3-MC, a prototypical AhR ligand. As expected, OME and TBZ were unable to displace [(3)H]-TCDD from its binding sites to the AhR in competitive binding studies. Moreover, classic tyrosine kinase inhibitor herbimycin A (HA) inhibited the two benzimidazoles-mediated CYP1A1 inductions, but only partially inhibited the 3-MC-mediated one. Another two tyrosine kinase inhibitors, Lavendustin A (LA) and genistein (GEN), had no effect on CYP1A1 induction by benzimidazoles and 3-MC. These results are consistent with the implication of a tyrosine kinase, most probably the Src tyrosine kinase, in the mechanism of CYP1A1 induction in rat hepatocytes.

Anticancer therapeutic potential of soy isoflavone, genistein

Genistein (4'5, 7-trihydroxyisoflavone) occurs as a glycoside (genistin) in the plant family Leguminosae, which includes the soybean (Glycine max). A significant correlation between the serum/plasma level of genistein and the incidence of gender-based cancers in Asian, European and American populations suggests that genistein may reduce the risk of tumor formation. Other evidence includes the mechanism of action of genistein in normal and cancer cells. Genistein inhibits protein tyrosine kinase (PTK), which is involved in phosphorylation of tyrosyl residues of membrane-bound receptors leading to signal transduction, and it inhibits topoisomerase II, which participates in DNA replication, transcription and repair. By blocking the activities of PTK, topoisomerase II and matrix metalloprotein (MMP9) and by down-regulating the expression of about 11 genes, including that of vascular endothelial growth factor (VEGF), genistein can arrest cell growth and proliferation, cell cycle at G2/M, invasion and angiogenesis. Furthermore, genistein can alter the expression of gangliosides and other carbohydrate antigens to facilitate their immune recognition. Genistein acts synergistically with drugs such as tamoxifen, cisplatin, 1,3-bis 2-chloroethyl-1-nitrosourea (BCNU), dexamethasone, daunorubicin and tiazofurin, and with bioflavonoid food supplements such as quercetin, green-tea catechins and black-tea thearubigins. Genistein can augment the efficacy of radiation for breast and prostate carcinomas. Because it increases melanin production and tyrosinase activity, genistein can protect melanocytes of the skin of Caucasians from UV-B radiation-induced melanoma. Genistein-induced antigenic alteration has the potential for improving active specific immunotherapy of melanoma and carcinomas. When conjugated to B43 monoclonal antibody, genistein becomes a tool for passive immunotherapy to target B-lineage leukemias that overexpress the target antigen CD19. Genistein is also conjugated to recombinant EGF to target cancers overexpressing the EGF receptor. Although genistein has many potentially therapeutic actions against cancer, its biphasic bioactivity (inhibitory at high concentrations and activating at low concentrations) requires caution in determining therapeutic doses of genistein alone or in combination with chemotherapy, radiation therapy, and/or immunotherapies. Of the more than 4500 genistein studies in peer-reviewed primary publications, almost one fifth pertain to its antitumor capabilities and more than 400 describe its mechanism of action in normal and malignant human and animal cells, animal models, in vitro experiments, or phase I/II clinical trials. Several biotechnological firms in Japan, Australia and in the United States (e.g., Nutrilite) manufacture genistein as a natural supplement under quality controlled and assured conditions.

A high-throughput cell-based reporter gene system for measurement of CYP1A1 induction

INTRODUCTION

Enzyme induction is undesirable in new drug discovery process, with consequences spanning from auto-induction to toxicity. Cytochrome P450 (CYP) 1A1 has long been known to be one of the metabolic enzymes involved in activating many procarcinogens, the first step toward tumor formation during chemical carcinogenesis. Induction of CYP1A1 during drug treatment may predispose the patients to some risk of chemical carcinogenesis.

METHODS

Based on the signal-transduction mechanism of CYP1A1 induction, a high-throughput reporter-gene system was established by stable transformation of H4IIE cells to incorporate the luciferase gene under control of CYP1A1 promoter. This stable cell line was validated with known CYP1A1 inducers, such as 3-methylcholanthrene (3-MC), beta-naphthoflavone (beta-NF), alpha-naphthoflavone (alpha-NF) and 3-indocarbinol. Thirty in-house new chemical entities (NCEs) were then screened with this reporter-gene system, and also administered to rats to evaluate in vivo CYP1A1 induction.

RESULTS

CYP1A1 reporter gene system can be used to identify strong inducers, such as 3-MC, beta-NF and alpha-NF, and weak inducers, such as 3-indocarbinol. In vitro induction of 30 in-house compounds in reporter gene system did not correlate with in vivo induction in rat liver microsome measured by ethoxyresorufin-O-dealkylation (EROD) activity, but had a reasonable correlation with Western blot signals.

DISCUSSION

This reporter-gene system may be useful in eliminating compounds that can cause CYP1A1 induction at an early stage of drug discovery.

Black tea polyphenols, theaflavins, prevent cellular DNA damage by inhibiting oxidative stress and suppressing cytochrome P450 1A1 in cell cultures

Tea polyphenols have been demonstrated as chemopreventive agents in a number of experimental models. However, less is known about the mechanism of chemoprevention by black tea compared with that of green tea. Some beneficial properties of theaflavins, the black tea polyphenols, were investigated in the present study. Theaflavins showed inhibitory effects on H(2)O(2)- and tert-butyl hydroperoxide (tBuOOH)-induced cytotoxicity (evaluated by tetrazolium bromide reduction), cellular oxidative stress (detected by oxidation of 2', 7'-dichlorofluorescin), and DNA damage (measured by amount of 8-OHdG and comet assay) in rat normal liver epithelium cell RL-34 cell lines. In addition, theaflavins also exhibited suppression of cytochrome P450 1A1 induced by omeprazole in the human hepatoma HepG2 cell line. Furthermore, when HepG2 cells were pretreated with omeprazole to induce CYP1A1, then exposed to benzo[a]pyrene (B[a]P), DNA damage was observed using the comet assay. However, theaflavins could inhibit this DNA damage. These results indicated that theaflavins could prevent cellular DNA damage by inhibiting oxidative stress and suppressing cytochrome P450 1A1 in cell cultures.

Induction of Cytochrome P450 1A1 in Mice by Repeated Oral Administration of Propranolol

Pretreatment of adult male C57BL/6 mice with propranolol (PL, 100 mg/kg, p.o., once a day for five days) significantly increased PL N-deisopropylase activity and decreased PL 7-hydroxylase activity in liver microsomes, whereas PL 4- and 5-hydroxylase activities remained unchanged. In the present study, we have examined the mechanism for the elevation of the oxidation of PL side chain. Immunoblot analysis using polyclonal antibodies raised against rat liver CYP enzymes such as CYP1A1, -2B2, -2C11, -2D2, -2E1 and 3A2 showed that, compared with the vehicle-treated control, the levels of two protein bands (54 KD and 52 KD) were increased by the pretreatment. Both proteins immunochemically cross-reacted with the antibodies against rat CYP1A1, and from their molecular weights, the 54 KD and 52 KD proteins were deduced to be CYP1A1 and 1A2, respectively. Computer-assisted scanning analysis revealed that the levels of CYP1A1 and CYP1A2 proteins were increased 1.8 and 1.2 times, respectively, over those of control microsomes. PL N-deisopropylase activity correlated well with ethoxyresorufin O-deethylase (r=0.828) and phenacetin O-deethylase (r=0.851) activities in the same microsomal fractions. These results show that repeated oral administration of PL in mice induces mainly CYP1A1 and also CYP1A2 to some extent, which contrasts from our previous results in rats in which CYP1A2 only was induced with PL pretreatment [Narimatsu et al., Chemico-Biol. Interact., 101, 207-224 (1996)].

Tyrphostin [correction of Tryphostin] AG879, a tyrosine kinase inhibitor: prevention of transcriptional activation of the electrophile and the aromatic hydrocarbon response elements

To investigate a possible role of phosphorylation in the signal transduction pathways responsible for transcriptional regulation of drug-metabolizing enzymes, we tested seven specific tyrosine kinase inhibitors (tyrphostins) for their effects on NAD(P)H:quinone oxidoreductase-1 (NQO1) mRNA levels in mouse hepatoma Hepa-1c1c7 (Hepa-1) cells and chose to study AG879 further. The potent electrophile tert-butylhydroquinone (tBHQ) is known to activate NQO1 gene transcription via the electrophile response element (EPRE). Among the tyrphostins tested, tyrphostin AG879 was unique in preventing the accumulation of tBHQ-induced NQO1 mRNA; this effect was dependent on the AG879 dose and was also sensitive to the time when AG879 was added relative to the beginning of tBHQ treatment. 2,3,7,8-Tetrachlorodibenzo-p-dioxin (dioxin; TCDD) is known to activate Cyp1a1 gene transcription by way of aromatic hydrocarbon response elements (AHREs). We found that AG879 also prevents, to a lesser extent, the AHRE-mediated induction of CYP1A1 and NQO1 mRNA by dioxin. Zinc or cadmium is known to activate metallothionein (Mt1) gene transcription via the metal response element (MRE). AG879 induced MT1 mRNA, and AG879 did not block zinc- or cadmium-induced MT1 mRNA, indicating that the effects of AG879 on NQO1 or CYP1A1 mRNA levels cannot be generalized to all transcripts. Using transient transfection of EPRE-, AHRE-, or MRE-driven luciferase reporter gene constructs in Hepa-1 cells, we showed that the inhibitory effects of AG879 occurred at the level of EPRE- and AHRE-mediated transcription, but that AG879 did not affect the MRE-driven transcriptional response. These data suggest that AG879 might inhibit an unknown tyrosine kinase(s) whose activity is essential for EPRE- and AHRE-mediated trans-activation of certain mammalian genes. These results also indicate that some sharing of common signal transduction pathways might exist in the regulation of genes involved in drug metabolism that also respond to oxidative stress.